The present invention relates to a DC motor having field poles each including a magnet pole of a permanent magnet and an auxiliary pole of a magnetizable material of high permeability such as soft steel.
As disclosed, for example, in Japanese patent publication No. 48-35721 published on Oct. 30, 1973, a DC electric motor having field poles of a permanent magnet is known in which the permanent magnet of each pole is partially replaced by an auxiliary pole of a magnetizable material of high permeability, such as soft steel to thereby provide the DC motor with series-wound characteristics. In the DC motor of this type, each auxiliary pole is disposed at the side of the associated field pole on which the armature reaction acts to produce magnetization effect, i.e. to increase the magnetic fluxes flowing therethrough, or in other words each auxiliary pole is disposed at the entry side of the associated field pole in the direction of rotation of the motor. Therefore, when the motor is reversely rotated, each auxiliary pole is disposed at the side of the associated field pole on which the armature reaction acts to produce demagnetization effect, i.e. to decrease the magnetic fluxes flowing therethrough so that the torque produced by the motor becomes smaller as the armature current becomes higher. Accordingly, the motor of this type is practically used for rotation only in one direction.
The above Japanese patent publication No. 48-35721 discloses a specific example in which the auxiliary pole is disposed at the side of the associated field pole on which the armature reaction acts to provide the demagnetization effect. However, this example is relating to a DC generator to be used as an electric power source for welding and aimed at providing the DC generator with a drooping characteristic of its voltage with respect to its load current, unlike the DC motor of the present invention having series-wound characteristics.